1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to an electronic system having common mode voltage range enhancement.
2. Description of the Related Art
Electronic systems often include amplifiers to amplify one or more signals. FIG. 1 depicts an embodiment of an electronic data acquisition system 100 that includes an amplifier 102. Signal source is, for example, a transducer that senses a physical event, such as a seismic event, and generates differential input signals VINS+ and VINS− that represent the physical event. Signal source 103 is connected to amplifier 102 via cables 108 and 109. In seismic exploration environments, the physical events can be the detection of ground vibrations. Additionally, in some applications, such as in a seismic exploration environment, cables 108 and 109 can be, for example, 40 ft-150 ft long. Electrostatic and/or electromagnetic sources in the vicinity of cables 108 and 109 can induce an undesirable common mode voltage VCM in the cables 108 and 109. The differential input signals VINS+ and VINS− are then superimposed on the common mode voltage VCM to generate respective input signals VIN+ and VIN−. Thus, the input signal VIN+ includes two components, i.e. signal VINS+ and the common mode voltage VCM. Likewise, input signal VIN− includes two components, i.e. signal VINS− and the common mode voltage VCM. Input signal VIN+=VCM+VINS+, and input signal VIN−=VCM+VINS−. Because input signals VINS+ and VINS− are differential signals, VINS+=−VINS−, i.e. signal VINS+ is an opposite polarity equivalent of signal VINS−.
Amplifier 102 receives input signals VIN+ and VIN−. Amplifier 102 is generally designed to reject the common mode voltage VCM, amplify the differential input signals VINS+ and VINS−, and generate differential output signals VOUT+ and VOUT−. In at least one embodiment, VINS+=−VINS− and VOUT+=VOUT−. The output signals VOUT+ and VOUT− can be used in many applications, such as input signals to an analog-to-digital converter (ADC) 104. The ADC 104 generates a digital output signal y(n) that represents the difference between input signals VIN+ and VIN−. Amplifier 102 operates from two, fixed supply voltage rails VH+ and VH−. A fixed voltage supply 106 generates the two fixed, supply voltage rails VH+ and VH−. The supply voltage rails VH+ and VH− are set to accommodate the maximum and minimum excursions of VIN+ and VIN−.
FIG. 2A depicts an exemplary voltage plot 200A associated with operation of amplifier 102 and common mode voltage VCM. Referring to FIGS. 1 and 2A, exemplary input signals VIN+ and VIN− vary over time and have respective maximum and minimum voltages of maxVIN and minVIN. “VIN” represents VIN+ and VIN− unless otherwise indicated. To prevent distortion of output signal VOUT+ and VOUT−, the values of fixed supply voltages VH+ and VH− are set so that VH+ is at least equal to the greater of (maxVCM+VIN—PP/2), and VH− is less than or equal to (minVCM−VIN—PP/2). VIN—PP is the peak-to-peak voltage of input signals VIN+ and VIN−. “maxVCM” is the maximum anticipated value of the common mode voltage VCM, and minVCM is the minimum anticipated value of the common mode voltage VCM. The supply voltages VH− and VH− are set to account for both the maximum and minimum excursions of input signal voltages VIN+ and VIN−.
FIG. 2B depicts a time varying voltage plot 200B of input signals VIN+ and VIN− having common mode voltage VCM superimposed on respective differential signals VINS+ and VINS−. In at least one embodiment, the maximum and minimum values of common mode voltage VCM is several times larger than the peak voltages of differential input VINS+ and VINS−. Power must be allocated to accommodate the common mode portion of the input signals to amplifier 102. For example, the common mode voltage can be relatively large, such as −10V<VCM<+10V and the differential signals VINS+ and VINS− have voltages that can be relatively small, such as +/−100 millivolts to, for example, +/−2.5 V. For a +/−10 V maximum/minimum common mode voltage VCM and +/−2.5 V maximum/minimum differential signals VINS+ and VINS−, the power requirements of amplifier 102 are P=I·(+/−12.5 V). “P” is the power supplied by fixed voltage supply 106, and “I” is the current supplied by fixed voltage supply 106.
Supplying power to accommodate the common mode voltage VCM when the differential signals are relatively small is generally inefficient and requires components with higher operational power limits. Higher operational power components generally trade off cost with accuracy.